1. Field of the Invention
This invention relates to a three dimensional laser velocimetry technique capable of measuring three components of velocity of a particle in a fluid flow.
2. Description of the Related Art
Optical techniques are desirable for the measurement of particle or fluid velocities as other instruments, such as physical type probes, disturb the flow at precisely the location of measurement. The non-contacting nature of laser beams has been used to measure the speed of particles at a localised region in fluid flows. The localised region in which measurement occurs is known as the probe volume and is usually characterised by focused laser beams.
The velocities of particles in a fluid flow have been remotely measured using a technique which involves the detection of laser radiation scattered from the particles suspended in the fluid as they traverse through the laser beams representing the probe volume. This technique utilises the doppler effect. Two beams of laser radiation undergo changes in their frequency when they are scattered or reflected off a moving particle suspended in the flow. The doppler difference frequency between the two scattered beams is detected as the frequency output from a photodetector. Knowledge of the doppler difference enables the velocity of the particles suspended in the flow to be calculated.
Three dimensional laser velocimetry techniques which utilise the doppler effect are known. One such technique is based on an optical measurement region containing three fringe patterns which are generated by the interference of six coherent beams of radiation. Each set of fringes is generated by the interference of two coherent beams of radiation of the same wavelength. The three fringes patterns are each of different wavelength and each is used to determine a single velocity component of a particle in the fluid flow. The three optical fringe patterns enable the three dimensional velocity vector of a particle in the fluid flow to be determined.